The present invention relates to a stripline filter device with means for adjusting a coupling quantity between pairs of dielectric substrates.
Such a stripline filter device is known, which is used as a band-pass filter for a microwave range. The stripline filter is becoming watched because it is easily made thinner or smaller than block type filters and suitable for meeting a requirement for a size reduction of potable communication equipments such as potable telephones.
An example of such a conventional stripline filter device is illustrated in FIG. 1. As will be seen in FIG. 1, a plurality of stripline resonator conducting layers A are disposed in an interdigitated form on a dielectric substrate B whose outer surface is provided with a ground conductor layer C with which one end of each resonator conducting layer A is connected. An example of such a stripline filter is disclosed in U.S. Pat. No. 4,157,517.
With this conventional stripline filter device, if the distance W between the adjacent resonator conducting layers A and A on the dielectric substrate B is too small, a coupling therebetween becomes strong to give too large resonance bandwidth. If the distance W between the adjacent resonator conducting layers A and A is set to a larger size so as to obtain a desired resonance bandwidth, the filter completed increases in size. In order to overcome this problem it has been proposed to provide a groove between the adjacent resonator conducting layers for forming a gap therebetween which decreases the quantity of the coupling. In this connection, for example, Japanese Patent Kokai Nos. 61-100002 and 61-201501 are referred. However, this method has a disadvantage that troublesome machining is required.